The present invention relates to a magnetic recording medium and a process for producing the magnetic recording medium, and more particularly, to a magnetic recording medium suitably applicable to existent magnetic recording systems using a ring-type magnetic head and capable of using a plastic substrate for providing a magnetic recording medium for high-density recording. Still more particularly, the present invention relates to a magnetic recording medium having an excellent stability independent of change in passage of time, a higher coercive force and a higher saturation magnetization, exhibiting a lower noise and a large output in a high frequency range, and being free from the distortion in reproduction waveform, and a process for producing such a magnetic recording medium in industrially and economically advantageous manner.
In recent years, in magnetic recording apparatuses such as a hard disc drive, there has been a remarkable tendency that information devices or systems used therefor are miniaturized and required to have a high reliability. In association with such a recent tendency, in order to deal with a large capacity data, there has been an increasing demand for providing magnetic recording media on which information can be stored with a high density.
In order to fulfill such requirements, the magnetic recording media have been strongly required to have an excellent stability independent of change in passage of time and a coercive force as high as possible and a saturation magnetization as high as possible.
As magnetic recording media having a high coercive force and a high saturation magnetization, there have been widely known those comprising a substrate and a magnetic thin film formed on the substrate.
The magnetic thin films which have been already put into practice, are generally classified into oxide thin films composed of maghemite, etc. (refer to "Technical Report of Electronic Telecommunication Institute", published by Electronic Telecommunication Institute, (1981) MR81-20, pp. 5 to 12), and alloy thin films composed of Co-Cr alloy or the like.
The magnetic oxide thin films composed of maghemite are excellent in oxidation resistance or corrosion resistance because maghemite is iron oxide. Therefore, the magnetic oxide thin films composed of maghemite can show an excellent stability independent of change in passage of time, and less change in magnetic properties with passage of time. However, the coercive force of the magnetic oxide thin films composed of maghemite is as low as at most about 700 Oe, and the saturation magnetization thereof is as low as at most 240 emu/cm.sup.3.
Whereas, the alloy thin films have a coercive force as high as not less than about 2,000 Oe, and a saturation magnetization as high as about 300 emu/cm.sup.3. However, metal materials themselves in the alloy thin films tend to be readily oxidized, so that the stability independent of change in passage of time is deteriorated.
In order to prevent magnetic properties of these alloy thin films from being deteriorated due to the oxidation, the surfaces of the alloy thin films have been coated with a protective film, e.g., a carbon film having usually a thickness of about 100 to about 200 .ANG.. However, in such a case, a whole thickness of the resultant magnetic recording medium becomes increased by the thickness of the carbon film, the loss due to the magnetic spacing (distance between a magnetic head and a magnetic recording layer) becomes large, resulting in incompetence of which disables the magnetic recording medium to be applied to high-density recording.
In consequence, it has been attempted to enhance the coercive force and saturation magnetization of the above-mentioned magnetic oxide thin films composed of maghemite which are excellent in oxidation resistance, corrosion resistance and stability independent of change in passage of time, by incorporating cobalt or the like into the maghemite thin film. The maghemite thin films containing cobalt, etc., have been already put into practice (Japanese Patent Publications (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993), "CERAMICS", published by Japan Ceramics Institute (1986), Vol. 24, No. 1, pp. 21-24, and the like). In addition, there have been extensively developed so-called perpendicular magnetic films which are capable of high-density recording because of free from demagnetization since these films are magnetized in the direction perpendicular to the surface thereof, and in which the plane (400), is predominantly oriented in the direction parallel with the surface of a substrate (Japanese Patent Publication (KOKOKU) No. 7-60768(1995) and Japanese Patent Application Laid-Open (KOKAI) No. 7-307022(1995)).
In the cobalt-containing maghemite thin films, the more the cobalt content, the higher the coercive force thereof becomes. However, with the increase in the cobalt content, the stability independent of change in passage of time of the magnetic maghemite thin films tend to be deteriorated by adverse influences of heat or the like.
In the case where information recorded on the above-mentioned perpendicular magnetic films is read out using an existent magnetic recording system equipped with a ring-type magnetic head which have been currently extensively used, there is caused the distortion of isolated reproduced waveform (i.e., an isolated reproduced pulse shape of so-called "di-pulse"). For this reason, it is necessary to conduct a specific signal processing.
In order to avoid conducting the specific signal processing, a single-pole magnetic head used in a perpendicular magnetic recording system may be used appropriately. However, the perpendicular magnetic recording system has not been extensively used yet and the single-pole magnetic head is still very expensive.
Thus, at the present time, there has been a strong demand for providing magnetic recording media for high-density recording which are suitable for use in current magnetic recording systems equipped with a ring-type magnetic head. For this reason, various kinds of magnetic recording media as mentioned above have been extensively developed. Among them, the magnetic oxide thin films composed of maghemite having excellent oxidation resistance and corrosion resistance have been considered to be most useful, so that it have been more strongly demanded to improve properties of these magnetic oxide thin films composed of maghemite.
That is, as described above, the cobalt-containing maghemite thin films have been strongly required not only to show a high coercive force and a high saturation magnetization, but also to exhibit a less noise and a large output in a high frequency region and to be free from the distortion of isolated reproduced waveform.
Meanwhile, the cobalt-containing maghemite film has been produced on a substrate by a sputtering method or the like. The production process generally requires a temperature as high as not less than about 300.degree. C. As a result, there arises a problem that only aluminum discs, aluminum alloy discs or the like which can withstand a temperature as high as not less than 300.degree. C., are usable in the process.
The conventional production processes in which the temperature as high as not less than 300.degree. C. is required, have such a problem that it becomes difficult to produce the cobalt-containing maghemite film in industrially and economically useful manner.
As conventional methods for producing the cobalt-containing maghemite thin film, there are known (1) a method of sputtering an Fe-Co alloy target in a mixed gas atmosphere composed of argon and oxygen to form a cobalt-containing hematite thin film on a substrate, reducing the hematite thin film in a hydrogen atmosphere at a temperature as high as 230 to 320.degree. C. so as to transform the hematite thin film into a cobalt-containing magnetite thin film, and then oxidizing the obtained cobalt-containing magnetite thin film at a temperature of 290 to 330.degree. C.; (2) a method of sputtering sintered magnetite containing cobalt as a target to form a cobalt-containing magnetite thin film on a substrate, and then oxidizing the obtained cobalt-containing magnetite thin film at a temperature as high as not less than 320.degree. C.; (3) a method of forming a cobalt-containing maghemite thin film directly on a substrate by decomposing a vapor of metal chelate, metal carbonyl or ferrocenes containing iron and M.sub.x (wherein M represents at least one element selected from the group consisting of Co, Cu, Rh, Ru, Os, Ti, V and Nb, and x is 0.01 to 0.1), and an oxygen gas in a pressure-reduced plasma which is highly-densified by applying a magnetic field thereto (Japanese Patent Application Laid-Open (KOKAI) No. 3-78114(1991)); or the like.
At the present time, as magnetic recording media for high-density recording which are suitable for use in current magnetic recording systems using a ring-type magnetic head, there have been demanded magnetic recording media having an excellent stability independent of change in passage of time, a high coercive force and a high saturation magnetization, exhibiting a lower noise and a large output in a high frequency region, and being free from the distortion of isolated reproduced waveform, and further it has been demanded to produce these magnetic recording media in industrially and economically advantageous manner without limitation to materials for substrates thereof. However, there have not been provided yet such magnetic recording media satisfying all of the above-mentioned requirements.
The conventional Co-containing maghemite thin films are still unsatisfactory in magnetic properties, especially cannot exhibit a high coercive force. When it is attempted to obtain a high coercive force, especially not less than 2,000 Oe, it is necessary to incorporate a large amount of cobalt thereinto. However, in such a case, the Co-containing maghemite thin film tends to be deteriorated in magnetic properties with passage of time by adverse influences of heat or the like.
Further, the cobalt-containing maghemite thin films have a problem that the output in a high frequency region is not larger for high-density recording.
As described above, the conventional perpendicular magnetic films composed of cobalt-containing maghemite have a problem concerning distortion of isolated reproduced waveform. Therefore, the perpendicular magnetic films cannot be applied to current magnetic recording systems using a ring-type magnetic head.
In the above-mentioned methods (1) and (2) of producing a cobalt-containing maghemite thin film, since these methods require a temperature as high as not less than 290.degree. C., a material usable for the substrate is restricted. Therefore, these conventional methods are industrially and economically disadvantageous.
In the above-mentioned method (3) of producing a cobalt-containing maghemite thin film, since the maghemite thin film can be formed on a substrate maintained at a temperature as low as about 50.degree. C., there can be used substrates composed of a plastic material having a low heat resistance, such as polyesters, polystyrene terephthalate, polyamides or the like. However, the coercive force value of the obtained magnetic recording media is about 1,700 Oe at most.
As a result of the present inventors' earnest studies, it has been found that by forming a nickel oxide underlayer on a plastic substrate by a reactive sputtering method; forming the cobalt-containing magnetite thin film on the nickel oxide underlayer by reactive sputtering a cobalt-containing iron alloy target at a substrate temperature of less than 240.degree. C. in a mixed gas atmosphere containing oxygen and a rare gas; and oxidizing the obtained cobalt-containing magnetite thin film in a plasma-activated oxygen atmosphere containing a rare gas at a substrate temperature of less than 240.degree. C., thereby converting the cobalt-containing magnetite thin film into a cobalt-containing maghemite thin film, a magnetic recording medium which is capable of having a high coercive force even when the cobalt content is as low as possible, exhibits an excellent stability independent of change in passage of time, a higher coercive force, a higher saturation magnetization even when the cobalt content is as low as possible, a lower noise and a large output in a high frequency region, and is free from the distortion of isolated reproduction waveform, can be produced in industrially and economically advantageous manner without limitation to a material for the substrate thereof. The present invention has been attained on the basis of the finding.